Earlier this month came a scientific discovery a century in the making: the first direct detection of gravitational waves.
But while the US Laser Interferometer Gravitational Wave Observatory (LIGO) got the honour of proving more convincingly than ever before that, yes, gravitational waves are a thing, researchers in Europe are pushing forward with a very different type of gravitational wave detector—in space.
On Tuesday, the European Space Agency’s LISA Pathfinder mission, a technology demonstration mission paving the way for the launch of a full-size detector in the 2030s, enters what ESA calls its “science mode” for the first time. Only then can it properly show that it’s possible to build a space-based detector, which will be able to observe gravitational waves that ground-based detectors like LIGO can’t and open a window onto the very early days of our Universe.
“Everything’s been wonderful; it’s just worked,” said LISA Pathfinder project scientist Paul McNamara of the mission so far in a phone interview. “It’s really surprised everybody it’s worked so well.”
LISA Pathfinder is not intended as a gravitational wave detector itself. Its objective is to test the technology for future space-based detectors such as ESA’s proposed Evolved Laser Interferometer Space Antenna (eLISA), which would launch in 2034.
A detector like eLISA would look at lower frequencies on the gravitational wave spectrum than LIGO, such as signals from supermassive black holes moving much slower than what LIGO can detect (supermassive black holes can be millions of solar masses large; the pair of black holes behind the LIGO detection were around 30 solar masses each). McNamara explained that a space-based detector like this would be able to look much further out—10 times the LIGO detector’s find at 1.3 billion lightyears away—and so make observations from earlier epochs in the formation of the Universe.
“With LISA [LISA was the original name for eLISA], because we can look right back to the first stars, we can actually almost time-slice the Universe, and we can say, ‘What kind of objects do we see at different epochs in the history of the Universe?’” McNamara said. This could help build up a picture of how the Universe evolved and answer questions like why many galaxies have a black hole at their centre.
But first the technology has to be tested to show that it is possible to build such a detector. LISA Pathfinder involves a spacecraft housing two test masses—4.6cm gold-platinum cubes—in an orbit 1.5 million kilometres away around the first Lagrange point. In the LISA Pathfinder demonstration, the two cubes are 38cm apart. In the eLISA mission, they will be in separate spacecraft one million kilometres apart—that’s what’s needed to detect the tiny signatures of gravitational waves (we’re talking measurements in the region of a millionth of a millionth of a metre).
LISA Pathfinder released its two test masses last week, opening the eight fingers and rods that kept each cube in place during liftoff. “Having a two-kilogram cube rattling around in its housing during launch would basically destroy the spacecraft,” said McNamara.
When these launch locks opened, the team was excited to see that the cubes detached as planned—they had feared there’d be some adhesion. “That was the first major success,” McNamara added.
Until now, the cubes have been held in place in the spacecraft as it moves by a weak electrostatic force. But from Tuesday, they should be in complete free fall and the spacecraft will move around them. Full science operations will begin on March 1.
As of Tuesday, if all goes to plan, the test masses in the LISA Pathfinder spacecraft will be floating in free fall, the spacecraft moving around them thanks to tiny microthrusters.
“We measure the position of the spacecraft with respect to the cube, and then we control that distance to be constant by pushing the spacecraft away,” explained McNamara. “You need to do this because when you’re trying to measure gravitational waves you want something that’s free in space, in an inertial orbit. And then when a gravitational wave comes it moves space, it changes the inertial position and the distance between the two bodies [the test masses].”
The spacecraft is there to shield the test masses from the environment—mostly solar radiation from the Sun—so that researchers can get a clear reading. McNamara compared it to the suspension system that protects the LIGO detector’s mirrors from seismic activity on the Earth.
When LISA Pathfinder launched last year, of course, we didn’t have direct observational evidence that gravitational waves existed. But McNamara said it was never ESA’s intention that the eLISA mission would be the first to detect them and there was never a sense of competition between them and their colleagues working on ground-based detectors.
“We’re all in this together; we’ve all worked on ground-based and space-based detectors,” said McNamara, who has worked at LIGO in the past and spoke with obvious enthusiasm at seeing the theory he’s worked on for decades have its moment in the spotlight. “There’s no way a mission launching in 2034, eLISA, was going to be the first detector so I have zero envy that they were first—I’m just delighted that we’re now observing the Universe with gravity.”
eLISA is not a gravitational wave detector, he said, but a gravitational wave observatory. Gravitational waves essentially allow us to view the Universe in ways the electromagnetic spectrum never could.
There are key differences between LIGO’s ground-based detector and a space-based observatory like the one LISA Pathfinder is paving the way for. The LIGO finding doesn’t really affect the eLISA mission, McNamara said, because “we’re in a completely different frequency range.” He used the analogy of LIGO looking for X-rays and eLISA looking for infrared rays.
“I should say that LISA is a lot easier than LIGO,” McNamara added, noting that the LIGO measurements are to a much greater precision. “We’re eight order of magnitude less sensitive than LIGO, but we measure over millions of kilometres, and they measure over four kilometres.”
And one major advantage: They know of some sources that are emitting gravitational waves quite near the Earth that an observatory like LISA could detect, and are able to work out their gravitational wave signature from their mass and location. “So as soon as we turn LISA on, we have these sources which are emitting gravitational waves and we know exactly how to measure them,” McNamara explained. They should be able to immediately detect these sources and calibrate their instruments to them.
For now, McNamara is optimistic about LISA Pathfinder’s progress and said he is confident the mission will far exceed its requirements and prove that the technology to build a gravitational wave detector in space is feasible. “Everything is going so well that I’m almost certain—I’m 99 percent sure—that we’ll meet our requirements as soon as we turn the system into its science operation,” he said. “Then what we’ll do is we’ll start to improve it.”
Having dedicated much of his life working in gravitational waves, McNamara said he has faced derision in the past from people who initially thought gravitational waves might not exist, and then that even if they did, they wouldn’t be detectable. The LIGO detection was, for him, “the greatest thing; something I’ve been waiting on for my entire career.”
Projects like the proposed eLISA will harness the gravitational waves to make observations of the Universe that would otherwise not be possible, and McNamara was enthusiastic about the increased interest from other countries following the breakthrough detection.
“If everybody else wants to jump on the bandwagon, everybody else wants to build a detector, please do,” he said. “The more detectors we get the better it is.”